Grinding of non-circular workpieces (also known in the art as non-round workpieces), such as cam lobes, by means of grinding wheel presents difficulties not found in grinding operations of rotationally-symmetrical workpieces. The contact area between the workpiece and the grinding wheel is continuously changing as the grinding wheel passes around the perimeter/circumference of the workpiece. For example in the case of cam lobes, the contact area is considerably larger in the flank portion than in the nose portion or the base circle portion. Therefore, optimization of grinding method for non-circular workpieces offers several difficulties and the success often depends largely upon the experience of an operator. Many grinding iterations are often required to achieve a desirable result.
On most CNC cylindrical grinders used today, the grinding wheel moves horizontally with the feed velocity (infeed and retraction) synchronized with the workpiece rotational speed to achieve the required tool path. If the workpiece is rotated with a constant workpiece rotational speed, drastic changes of the instantaneous grinding conditions occur during a revolution of the workpiece due to its non-circular geometry. Therefore, the instantaneous workpiece rotational speed of the workpiece is suitably varied during a revolution of the workpiece to control and to reduce the effect of the changes in instantaneous grinding conditions.
Different strategies have been proposed for optimizing cam lobe grinding methods. Such strategies include constant stock removal rate or constant spindle power, both of which are commonly used in the industry today.
U.S. Pat. No. 6,808,438 B1 discloses a method in which the component is rotated through only one revolution during a finish grinding step and the depth of cut and the headstock velocity are controlled during the single rotation so as to maintain a substantially constant load on the grinding wheel spindle drive motor. The component speed is altered from one point to another during each revolution so as to maintain the constant load.
U.S. Pat. No. 7,153,194 B2 discloses a method of grinding a component, such as a cam, comprising rotating the component through only a single revolution during a final grinding step and controlling the depth of cut and the component speed of rotation during the single revolution, so as to maintain a substantially constant specific metal removal rate during the final grinding step. The depth of cut can be kept constant whereas the workpiece speed of rotation is altered during the final grinding step to accommodate any non-rotational features of a workpiece so as to maintain the constant specific removal rate.
The above described methods improve productivity compared to conventional grinding with constant workpiece rotational speed of the workpiece. However, grinding methods which achieves a constant stock removal rate or a constant spindle power do not consider workpiece surface temperature. Therefore, none of them provide any means for controlling thermal damage, which is one of the main limitations of the grinding method in terms of productivity and quality. In cases where thermal damage occurs, the general strategy by process planners has been to either decrease the wheel feed increment or to reduce the workpiece rotational speed, both of which will result in a grinding method which is not fully optimized, yielding cycle times longer than necessary. The process planners use the above described optimizations methods only to calculate the workpiece rotational speed for achieving constant stock removal rate or constant spindle power. However, the determination of number of increments and the depth of cut for each increment are still based on trial and error and selected subjectively by machine operators.
A new strategy for optimizing peripheral cylindrical grinding of non-circular workpieces was proposed by Krajnik et al., “Optimization of peripheral non-round cylindrical grinding via an adaptable constant-temperature process”, CIRP Annals—Manufacturing Technology 62 (2013) 347-350. In accordance with this strategy, the grinding is optimized by choosing process parameters based on a thermal model for achieving a constant maximum surface temperature of the workpiece with the purpose of reducing the grinding cycle time while avoiding thermal damage of the workpiece.
Krajnik et al., “Cycle optimization in cam-lobe grinding for high productivity”, CIRP Annals—Manufacturing Technology, available online 13 Apr. 2014, discloses further developments as to the strategy of optimizing a grinding process based on a thermal model. This article discloses for example that considerably lower grinding times per cam lobe compared to the processes of constant stock removal and constant spindle power can be achieved. It also discloses that the instantaneous specific energy into the workpiece depends on the aggressiveness number. Furthermore, the article discloses that minimizing the grinding time per cam lobe of the grinding method requires employing the optimal number of feed increments, which depends on the set temperature and the machine limitations.
While it is clear that the proposed strategy of controlling the process parameters with the purpose of maintaining a constant maximum surface temperature of the workpiece overcomes the problems associated with thermal damage, it is not clear how the strategy can be industrially implemented. Moreover, the grinding method using the proposed strategy of controlling the process parameters with the purpose of maintaining a constant maximum surface temperature of the workpiece does not necessarily result in an acceptable quality of the workpiece after grinding. In fact, it is not taught how to control the quality of the surface of the workpiece. Further development is therefore needed.